It is a model 506/F where the 5 denotes the abbreviated keyboard (keys up to 5), the 06 denotes the 6 columns of keys, and the F denotes a Sterling currency version with farthings (¼ penny) keys.

170 mm x 190 mm x 118 mm. (6.7" x 7.5" x 4.6").

One of the more minimalist calculators made. Has 2 decades of £ keys, shillings, pence, and farthings (quarters of a penny) keys, with each decade numbered from 1 to 5. To enter a number between 6 and 9 (or 11 in the case of the
pence column) two keys have to be pressed in succession in that column. The knob on the right is for clearing the total.The machine illustrated shows the total £12 17s 11 ¾d.

Made in England by Bell Punch Co. Ltd. and distributed by Sumlock Compometer Ltd.

The machine illustrated has serial number 506/F/45.74, and has the older, rounded style casing. The farthing ceased to be legal currency in Britain in 1961, when this model would have become obsolete.

Close up of the keyboard and display, showing the total £12 17s 11 3/4d.

The bottom of the machine.

Many patents are listed on the label.The first UK patent is 504,457, where the application was made on October 20 1937 by John Fishburn of 39 St. James's Street, London S.W.1 (the address of Bell Punch's marketing
company).The other UK patent applications listed were made by Christopher Webb of the Bell Punch Company. The last is patent 652,670, where the application was made on September 23, 1948, and was published in 1951.

B.S. 1909 probably relates to British Standard 1909.

This machine is really capable only of adding lists of amounts to £999 19s 11 3/4d, but capable of doing this quickly and accurately (and cheaply !).

There are excellent photographs and descriptions of the Plus Adder mechanism at John Wolff's Web Museum site. These are accessed from the Comptometers and Key-Driven Calculators section on that site.